The development of the Internet and other similar distributed data communication networks (i.e., Internet Protocol (“IP”) based networks, or, simply, ‘IP networks’) has facilitated a growth in the number of people using IP-based networks to exchange various types of information, such as voice, fax and modem calls. The key method supporting fax calls over packet networks is the fax over IP (“FoIP”).
Fax relay is considered a reliable method for carrying out ‘end-to-end’ fax communications over IP networks. The communication protocol of FoIP systems is described in ITU-T Recommendation T.38 (“Procedures for real-time Group 3 facsimile communication over IP networks”) Roughly, ITU-T T.38 is a set of rules defining the FoIP call establishment and the packet format for transferring different signals of regular G3 and V 34 half-duplex (“HDX”) fax calls. The other fax relay protocol is defined in the Voice over Frame Relay Implementation Agreement FRF 11 but it has a limited use in comparison to T.38.
Traditionally, fax machines were, and still are, designed to make fax calls via a public switched telephone network (“PSTN”). The way in which such fax calls are managed and the way they traverse the PSTN infrastructure is defined in the ITU-T Recommendation T.30 (“Procedures for document facsimile transmission in general switched telephone networks”). Briefly, fax machines employ modulation schemes for allowing such traversals. However, for enabling FoIP calls, the signal output by a fax machine must be translated into a bit stream suitable for transmission via an IP network. The translation is performed by a corresponding media gateway (hereinafter “gateway”) that is connected on one hand to the fax terminal or PSTN, and, on the other hand, to the IP network. Such media gateways typically include a ‘fax relay’, speech coder and voice band data (“VDB”) paths for enabling fax calls, verbal and modem communication, respectively, as schematically shown in FIG. 1.
Traditionally, voice-over-IP (‘VoIP’) gateways process input and output voice band signals at 8 kHz sampling rate. Gateways exchange the signals with analog devices via interface circuits. Higher sampling rates are allowed as well However, for pulse code modulation (“PCM”) based transfer over IP, which is based on bit rates up to 64 kbps (kilo-bits-per-second), the signals should be sampled at a rate of 8 kHz. Typically, the input samples represent a sine waveform (carrier) signal modulated by a digital bit stream according to a standard modulation scheme. The modulating bit stream may contain some useful information such as fax capabilities, commands, responses, image bit map, transferred binary file, mixed raster content, etc
A purpose of fax gateways is to deliver all or most of the signals end-to-end between two communicating fax machines by a way that the faxes are able to complete a fax call/session without error alarms and such that an answering fax machine may receive analog signals from a gateway and extract a binary information transferred to it by the ‘In-Message’ procedure, and interpret it as intended by the originating fax.
Typically, fax relay gateways detect, demodulate and re-modulate all types of fax signals from the beginning till the end of a fax call. Usually, if the gateway is unable to relay at least some fax signals then it employs the VBD method to transfer a complete call over IP. It is noted that by using the wording ‘VBD method’ and ‘fax relay’ it is also meant to include the use of corresponding software tools, hardware, algorithms and applications as required for operating these circuitries.
By ‘VBD’ is meant method according to which modem's signals are transported over IP networks using a codec that passes voice band data modulated signals with minimum distortion. Exemplary VBD codecs are the G.711 A-law and G 711 μ-law codecs, which are commonly utilized by ITU-T V.150.1 (“Procedures for the end-to-end connection of V-series DCEs over an IP network”) and ITU-T V.152 (“Procedures for supporting Voice-Band Data over IP Networks”) standards PCM encoders of the G.711 type output bit streams at the rate 64 kbps, whereas PCM encoders of the G.726 type output bit streams at a rate of up to 40 kbps.
Typical FoIP media gateways, readily available on the market, were primarily designed to cope with (regular) group 3 (“G3”) fax machines, which are capable of producing HDX fax image data streams at a bit-rate less than or equal to 144 kbps, and exchanging HDX fax's control signals at a rate of 300 bps. After being received by a gateway, these signals are handled by the fax relay. The fax signal translation performed in the gateway involves demodulation of the incoming sampled signal by a fax relay receiver, packetization of the demodulated data according to a fax relay protocol (‘FRP’), and transmittal of the FRP packets to the IP network. A receiving gateway includes a fax relay transmitter, the task of which is to re-modulate the FRP packets, received from the packet network, according to modulation schemes that are defined for G3-type faxes in the ITU-T Recommendation T.30
Currently, there exist V 34-type fax machines that are known to those skilled in the art also as “super-G3” fax machines, which have a signal flow and modulation schemes that are more advanced in comparison to those of regular G3 fax machines. For example, a super-G3 fax machine transfers HDX fax images at a bit-rate as high as 336 kbps (vs 14.4 kbps in G3 fax machines) and exchange different types of fax control signals full duplex at bit-rates of 300, 600 and 1200 (2400) bps. Full support of V.34 fax relay as defined by T.38 Rec. is highly problematic due to considerable computational resources that are required, on the receiving/transmitting gateway side, for processing V.34 fax images and training equalizer signals.
Traditional gateways avoid the latter obstacle by using the relatively low-complex VBD method. They do so by switching to VBD ‘mode of operation’, handling and transferring (i.e., over the IP network) the incoming V.34 type signals as if they were voice band modem signals; i.e., without demodulation. However, as far as operating under real IP network conditions, the latter solution is very problematic shorthand and unsatisfying; that is, IP networks have variable packet latency, multi-node packet processing and bandwidth restrictions. More specifically, the problems associated with the latter solution are the following:
i) Data streams exchanged by gateways are required to be full duplex at bit-rates of 64 kbps in order to allow communications at fax rates above 7.2 kbps;
ii) The ‘VBD path’ (i.e., in a gateway) is highly sensitive to residual echoes and imperfection of echo canceling process, which tend to cause ‘fallbacks’ in bit-rates, inaccurate reconstruction of the original fax images, and fax calls failures;
iii) The traditional solution involving usage of VBD is sensitive to packet latency. Long constant delays of packets in the IP network may cause signal collisions at the destination fax machine, and failure or abortion of fax sessions;
iv) The ‘VBD path’ is highly sensitive to packet jitter (variable delay) in the network. An attempt to extend the input queue buffers for solving this problem would result in large constant delays, possible collisions of fax signals, and ‘timeout’ related disconnections, and
v) The ‘VBD path’ solution is highly sensitive to packet loss.
Therefore, as far as super-G3 fax terminal equipments are concerned, traditional gateways provide a solution that is far from being adequate.